1. Field of the Invention
The present invention generally relates to a method and device for contactless inspection of objects on a substrate, during relative motion between the substrate and the inspection device.
2. Description of Background Art
When arranging objects on a substrate geometrical and other properties of the objects are important for the performance of the resulting product. Consequently, it is desired to be able to quickly and accurately perform an automatic inspection of these properties. Geometrical properties can for example be volume, position on the substrate, diameter, shape of outline, scratches, surface roughness etc. Other properties can be, colour, etc. An automatic inspection of the properties is difficult to perform at high speed and high accuracy. For example, in the process of applying solder paste to the substrate, by dispensing it or the like, the properties of the resulting solder paste deposit, e.g. volume and position, are important for the subsequent process steps and final yield.
Prior art is generally based on different imaging technologies such as 2-dimensional image processing, pattern recognition and/or 3-dimensional optical triangulation, stereo photography, moirxc3xa9 methods and white-light interferometry.
For obtaining height information of an object, laser triangulation is often used, such as in an apparatus and a method for inspecting solder printing as disclosed in U.S. Pat. No. 5,134,665. A radiation source, generally a laser, is positioned at a lateral distance from a sensor and illuminating the object to be inspected from one direction. The object is imaged onto the sensor via a radiation focusing element, such as refractive optics. The most common triangulation methods employ illumination with a single spot, a sheet-of-light or multistrip-light triangulation. The sensor views the object from another direction than the object is illuminated from and, thus, detects radiation reflected or reemitted from the object. Since the sensor is two-dimensional and since the positions of the radiation source and the sensor and the base plane for the object are known, it is possible to determine the height of the object by determining the direction of the radiation incident on the sensor.
Additionally, by scanning the whole object and determining a large number of height points or height profiles it is possible to determine an approximate volume of the object.
However, there are problems associated with prior art methods and devices. It is desired to combine speed and flexibility in one and the same arrangement. Generally, prior art methods and devices are dedicated to a single task and often they are not fast enough to comply with current and future demands.
In the above mentioned U.S. Pat. No. 5,134,665 there is disclosed an apparatus for inspecting solder paste prints on a printed circuit board (PCB), which is one type of substrate. Other types of substrate are for example substrates for ball grid arrays (BGA), chip scale packages (CSP), quad flat packages (QFP), and flip-chips. The apparatus measures print deviation, film thickness and print pattern of solder paste printed on pads formed on the PCB. A height measurement is performed by means of a laser ray illuminating the PCB pointwise. By mutually moving the apparatus and the PCD the laser point is scanned over a single solder paste object. By scanning the object in orthogonal X- and Y-directions the projection of the object is obtained in the form of an X-direction and a Y-direction profile line showing both the solder paste object and the underlying pad. By this known apparatus the positions and thicknesses of screen printed solder paste objects in relation to pre-printed pads are determinable. Drawbacks of this known apparatus are the limited use thereof. For example, neither accurate volume measurements nor accurate area measurements are performable, at least not reasonably fast, since this would require a vast number of scans in both directions.
Another solution dedicated for inspection of solder paste prints on a PCB is an apparatus manufactured by Philips called TriScan. The TriScan apparatus uses an advanced optical scanning system comprising a 20-side polygonal mirror rotating at a very high speed of up to 50 revolutions per second. A laser ray is projected onto the mirror and thereby a laser point is swept over the object at a rate of up to 1000 light sweeps per second. By advanced sets of mirrors the object is illuminated by said sweeps and reflected light is caught and guided to a sensor. While this apparatus enables several types of properties to be inspected at high speed it is complex and performs only measurements of height profiles as a basis for all determinations. The limitations to height profile measurements causes a limited accuracy. By measuring the profiles extremely closely, a certain improvement of accuracy may be obtained, However, this requires a high speed of measurement, which is difficult to achieve.
One object of this invention is to provide an inspection device for inspection of objects on a substrate during relative motion between the device and the substrate, and a method for inspection of objects on a substrate by means of such an inspection device, wherein said device and said method in an improved way combine inspection accuracy and a multiple task capability at high speed and low cost.
In one aspect the present invention relates to a method for contactless inspection of objects on a substrate, by means of an inspection device during relative motion between the substrate and the inspection device. The method comprises the steps of:
generating a first image comprising object height information by illuminating at least a portion of the substrate comprising one or more objects by means of first radiation means and imaging at least one of said one or more objects illuminated by said first radiation means onto a two-dimensional matrix sensor means having a portionwise addressable matrix of pixel elements;
generating a second image comprising object area information by illuminating at least a portion of the substrate comprising one or more objects by means of second radiation means and imaging at least one of said one or more objects illuminated by said second radiation means onto said sensor means;
extracting the object height information, by means of said sensor means, from said first image; and
extracting the object area information, by means of said sensor means, from said second image.
In another aspect the invention relates to a device for performing the above method. The device comprises a two-dimensional matrix sensor means having a portionwise addressable matrix of pixel elements; a first radiation means; a second radiation means; and imaging means for imaging radiation originating from an object plane onto the sensor means. Said first radiation means is arranged for illuminating at least a portion of the substrate comprising one or more objects, when the substrate is in said object plane, said imaging means thereby generating a first image of at least one of said one or more objects, said first image comprising object height information. Said second radiation means is arranged for illuminating at least a portion of the substrate comprising one or more objects, when the substrate is in said object plane, said imaging means thereby generating a second image of at least one of said one or more objects, said second image comprising object area information. Said sensor means comprises extraction means for extracting, from said first image, object height information, and for extracting, from said second image, object area information.
The generation of a first and a second image used for extracting object height information and object area information respectively in combination with employment of a matrix sensor means having a portionwise addressable matrix of pixel elements provides for an efficient and flexible use of the generated image information for inspecting and determining properties of the one or more objects. Characteristic for such a sensor means is the matrix of pixel elements and the possibility to address, and thus read out, only a portion of the whole matrix at a time. Portionwise is to be interpreted as at least one pixel at a time. This possibility is inventively employed to dedicate different combinations of pixel elements for different tasks. By generating two different images, by means of said first and second radiation means, the multitask capability is inventively and efficiently used.
In comparison to the above described device and method of U.S. Pat. No. 5,134,665 the present invention provides for both 2-dimensional and 3-dimensional inspection of objects more or less simultaneously while moving the substrate and the device in relation to each other.
The expression xe2x80x9cobjects on the substratexe2x80x9d comprise many different possible objects, such as for example adhesive, flux, conductive adhesive, soldered seams, electronic components, soldered electronic components, bumps, pins, and, particularly, deposits such as single or groups of solder paste or adhesive dots. The deposits may also comprise satellites, i.e. unwanted droplets of dispensed solder paste, adhesive, conductive adhesive, etc., from the dispensing process.
By xe2x80x9cradiationxe2x80x9d is meant different types of light, such as visible light, infrared light, ultraviolet light, etc.; and by xe2x80x9cfrequencyxe2x80x9d is meant the frequency of the radiation waves. Instead of xe2x80x9cfrequencyxe2x80x9d the term xe2x80x9cwavelengthxe2x80x9d could be equivalently used.
In an advantageous embodiment of the invention the sensor comprises on-chip signal processing capability. The signal processing consequently is performed on the same chip as the pixel elements are formed, which enhances the speed of the device by reducing the amount of output needed to be applied to external processing means.
In an advantageous embodiment of the invention said first and second images are alternatingly processed portion by portion, i.e. the images are processed portionwise and the processing jumps to and fro between the two images. This way of processing is close to parallel processing of the height and area information respectively. By employing a particularly advanced sensor means, in a further embodiment of the invention, it is even possible to perform a true parallel, that is simultaneous, processing of the area and height information respectively.
In yet further embodiments of the invention processing of the first and second images are additionally separated.
One aspect of separation is to separate the generation of the first and second images in time, thereby to minimise a possible difficulty of radiation associated with one of the images interfering with radiation associated with the other image on the sensor in a case where the first and second images overlap on the sensor surface. This time separation rather provides for a use of the same sensor area for sensing both images, which is advantageous in some cases. Additionally, the possibility of illuminating the same area of the substrate without risking radiation interference in the object plane is enhanced.
Another aspect of separation is to separate the first image from the second image by having different, that is geometrically separated, portions of the sensor means illuminated by the first and the second image respectively. As a result the risk of interference is substantially eliminated, and by using, in this way, different sensor elements for the different images the total rate of images generated can be increased.
A further aspect of separation is to separate the first image from the second image by separating the radiation originating from said first and second radiation means in a first and a second range of frequencies respectively, and by at least filtering the radiation impinging a first portion of the sensor means so that radiation within one of said first and second ranges of frequencies is passed and radiation within the other one of said first and second ranges of frequencies is stopped. In addition to the above mentioned advantages of the enhanced separation, this aspect provides for a possibility to at least to some extent limit the area inspected on the substrate by limiting the dimensions of said first portion. Optionally, two or more portions of the sensor means are covered by filters passing radiation within different frequency ranges.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.